The LC/MS has been widely used in the field of chemical analysis. In an apparatus for conducting measurement by an LC/MS, a sample in a liquid state is ionized at atmospheric pressure and is then introduced into a mass spectrometer that is placed in a vacuum. One of the main ionization methods used for this purpose is an atmospheric pressure chemical ionization method (hereinafter abbreviated to "APCI"). An example of an APCI method is described in Japanese Patent Laid-Open No. 172245/1992.
FIG. 8 schematically shows a typical structure of an ion source used in an APCI method (hereinafter referred to as an APCI ion source). A sample in the liquid chromatograph 1 is introduced with a solvent as a mobile phase through a Teflon tube 2 and a stainless steel capillary 3 into a nebulization region 4 that is heated to 200 to 400.degree. C. The sample is nebulized in the nebulization region 4 and is then sent to a desolvation chamber 5 having a vaporization region. The desolvation chamber 5 is fixed to a first electrode 10 through an O-ring 40 for maintaining a vacuum seal. The desolvation chamber 5 is heated to about 400.degree. C., and this heat is used as the heat of vaporization of the sample 6.
Next, the sample 6 with solvent is sent into a corona discharge space having a needle electrode 8 to which a high voltage is applied by a power supply 7 for the corona discharge. A plate 27 is positioned to close a portion near the center of the passage through which the sample and solvent flow. The plate has orifices about its periphery and is disposed at the inlet of the corona discharge space in order to prevent large charged droplets from being directly introduced into ion intake orifices in the first electrode 10. In this corona discharge space, primary ionization of the solvent molecules occurs and the sample is subsequently ionized by the ion-molecule reaction.
The resulting ions 9 are sent into a differential pumping region 18 formed between the first and second electrodes 10 and 11 and pumped down by a pumping system 12, through the ion intake orifices. An ion acceleration voltage is applied to each of the first and second electrodes 10, 11 by respective power supplies 13, 14. The ions thus accelerated pass through the ion intake orifices in the second electrode 11, are then focused by an ion focusing lens 15 and are thereafter sent to a mass spectrometer, as shown by the arrow.
Besides an APCI ion source method using heating for nebulizing the sample, there is also a method that uses gas nebulization. The construction of a typical gas nebulization apparatus is shown schematically in FIG. 9. A sample in the liquid chromatograph 1 is introduced with a solvent through a Teflon tube 2 and a stainless steel capillary 3 in a mobile phase into a nebulization region 4 through which a nebulization gas 16 flows. After being nebulized at atmospheric pressure, the sample is sent into a desolvation chamber 5 through which a vaporization gas 17 flows. The desolvation chamber 5 is fixed to the first electrode 10 through an O-ring 40, which is made of a rubber for example, for maintaining a vacuum seal. The desolvation chamber 5 is heated to about 150.degree. C., and this heat is used as the heat of vaporization of the sample 6.
Next, the sample 6 with solvent is sent into the corona discharge space having a needle electrode 8 to which a high voltage is applied by the power supply 7. Primary ionization of the solvent molecules occurs in this corona discharge space, and the sample is subsequently ionized by the ion-molecule reaction. The resulting ions are sent into the differential pumping region 18 encompassed by the first and second electrodes 10 and 11 and pumped down by the pumping system 12, through ion intake orifices in the first electrode 10. An ion acceleration voltage is applied to each of the first and second electrodes 10, 11 from the respective power supplies 13, 14. The ions 9 are accelerated so as to pass through the ion intake orifices in the second electrode 11, are then focused by the ion focusing lens 15 and are thereafter sent to the mass spectrometer, as indicated by the arrow.